The present invention pertains generally to radar systems, and particularly to radar systems that use synchronous pre-pulsed anti-radiation missile decoys.
It is well known in the art that an anti-radiation missile (ARM) is adapted to home on radio frequency (RF) signals radiated so that an explosive charge carried by such a missile may destroy a radar. To accomplish such homing, the guidance system in an ARM missile may be designed to lock onto the leading or trailing edge of radar interrogating pulses, as well as midpulse samples of such pulses. It is therefore desirable that, in order to increase the chance of survivability of a radar attacked by an ARM missile, decoys located in the vicinity of the radar be actuated to generate RF signals to cause the guidance system in an attacking ARM missile to home on an apparent source spaced from the radar. Thus, RF signals from the decoys are synchronized with the interrogating signals from the radar so that the RF signals from the decoys produce pulses overlapping (in power and time) the interrogating pulses produced by sidelobes of the antenna in the radar. Consequently, the guidance system in an attacking ARM missile is inhibited from using the leading or trailing edges or midpulse samples of the radar""s interrogating pulses to accurately derive guidance commands. Further, the decoys are caused to xe2x80x9cblinkxe2x80x9d. That is to say, the position in time, relative to the defended radar of each of the decoys is periodically altered. As a result of such xe2x80x9cblinkingxe2x80x9d the aim point of the ARM is caused to wander, thereby preventing the ARM from homing on the radar or any one of the decoys. A system that alters the blinking time at the rate of the radar""s pulse repetition frequency is described in U.S. Pat. No. 4,990,919, issued Feb. 5, 1991 to Manoogian entitled MISSILE DECOY SYSTEM. This system, however, suffers from significant drawbacks, such as relying only on open-loop pulse-to-pulse switching of the lead decoy. Furthermore, the above system, which changes the lead decoy on a pulse-by-pulse basis, is not a viable technique to assure decoy survivability. Bias errors in any real anti-radiation missile (ARM) seeker will tend to drift toward one of the decoys because of the noise-like pulse-by-pulse switching. Thus, if the ARM has a bias of 1 millivolt xe2x80x9cUPxe2x80x9d and 1 millivolt xe2x80x9cRIGHTxe2x80x9d, after, a number of pulses (e.g. one thousand pulses) the ARM will have migrated or drifted to the up-most, right-most decoy eventually destroying it before homing in on the next target.
Other decoy blink and pre-pulse strategies use an open loop approach based on typical assumed threat guidance loop parameters or time constants and are not altered on a radar pulse-by-radar pulse basis. In these systems, the blink rate is approximately equivalent to one missile guidance loop time constant as estimated from simulations. An ability to monitor ARM range and range rate to provide an estimate of time-to-go (or time since maneuver) in the scenario in order to increase survivability of the radar and decoys is highly desired.
The present invention directly addresses the rate at which the lead decoy should be altered based on a closed loop measurement of the response of an ARM to the change in position caused when the leading decoy is changed. The present invention couples this information with the gross angle of arrival (a quadrant) and the known relative positions of the decoys to intelligently control the decoy blink rate in a closed loop approach that can be xe2x80x9ctunedxe2x80x9d to the missile guidance loop""s impulse response to induce the maximum miss distance constant with maximizing the impact point away from the defended radar. One approach to this is to monitor the ARM""s range rate in response to decoy blinking actions. Those actions producing the largest range rate differences as decoy blink rate is swept over small time increments should revert immediately to the previous condition of blink rate, effectively zeroing in on the blink rate producing the maximum missile maneuvers (indicated by the changes in range rate). This activity continues until the ARM impacts, indicated by the loss of target in the missile warning radar. At this point the decoy system may revert to its default open loop blink rate condition as determined by simulation.
The invention improves the probability of survival of radars defended by decoys as well as the probability of survival of the decoys themselves by incorporating the range, range rate and quadrant of attack of the ARM; information readily available from low cost missile warning radars.
The present invention is embodied in a method of protecting a pulse radar from a missile attempting to home on interrogating pulses emitted by such radar, the method comprising the steps of providing a plurality of decoys at different locations in the vicinity of the pulse radar, each one of the decoys being adapted, when activated, to emit pulses of a given amplitude, frequency and duration; sequentially activating a selected decoy in the plurality of decoys to be the leading decoy in time relative to the other decoys and the radar, to form a covering pulse overlapping the then emitted one of the interrogating pulses; and adaptively changing the blink rate in response to an associated maneuver change of the ARM in response thereto by incorporating outputs of a low cost missile warning sensor, to obtain the blink rate that provides the maximum maneuver change associated with the ARM.